1. Field of the Invention
The invention relates to silver-palladium alloys for the manufacture of dental prostheses which can be faced with dental ceramic.
2. Description of the Related Art
Fixed and removable dental prostheses are chiefly made of corrosion-resistant, biocompatible precious metal alloys; with the so-called lost wax process, wherein the cast object is often faced with dental ceramic in order to achieve an appearance corresponding to the natural tooth. For this purpose, the alloys must possess specific properties, such as coefficient of thermal expansion, melting range and adhesion, matched to the dental ceramic.
Alloys with a high gold content, such as are described in patent specifications DE-11 83 247 and DE-15 33 233, for example, are particularly well suited to these purposes. Because the price of gold is high and varies considerably, however, in more recent times attempts have increasingly been made to find more reasonably priced alternatives to the alloys with a high gold content. Of the precious metals, palladium is considered for use because of its relatively favourable price, its distinctly reduced density compared to gold and its corrosion and/or mouth resistance, which are comparable to gold.
In addition to a lower price, compared to alloys with a high gold content, palladium base alloys have a higher hardness and strength, a higher solidus temperature brought about by the palladium, a better high temperature resistance and hence, with fired-on alloys, a higher distortion resistance when the ceramic is fired. These alloys are either gold-free or contain only few wt. % of gold. They do, however, react more sensitively to processing errors and are difficult to solder. The palladium content leads to a reduction of the coefficient of thermal expansion, although this may be equalized by the addition of silver.
In respect of their processing behaviour, palladium base alloys containing silver should be classed between the palladium base alloys with a high gold content and the silver-free ones. Compared to silver-free palladium base alloys, because of the silver content, palladium base alloys containing silver can be melted and cast more satisfactorily, have a lighter oxide, exhibit good soldering behaviour, and are also even more reasonably priced.
A dental system which consists of an alloy with a high gold content, a low-melting dental ceramic, and the corresponding solder alloys, was introduced onto the market a few years ago. Compared to the ceramics which had been used up to that time, the low-melting ceramic has a substantially higher coefficient of thermal expansion.
There is an increasing desire to combine this low-melting dental ceramic with a more reasonably priced palladium-silver and/or silver-palladium alloy which can be fired on.
For palladium base alloys which can be fired on, however, a higher coefficient of thermal expansion of the ceramic means a distinct increase in the silver content, to approx. 50 wt. % and above.
The disadvantage of these silver base alloys and/or palladium base alloys with a high silver content is that they discolour the facing ceramic, yellow and/or yellowish green during the firing-on process. The reason for this is the silver which finds its way into the ceramic through diffusion and/or via the vapour phase.
The problems relating to the discoloration of ceramic by silver are known.
Solutions have been found (DE-39 05 987) for palladium-silver alloys which can be fired on for conventional high-melting ceramic with a coefficient of thermal expansion in the range from 14 .mu.m/mK to 15 .mu.m/mK, up to a silver content of 45 wt. % of silver. These alloys are not, however, suitable for firing on low-melting ceramic with a coefficient of thermal expansion of approx. 16.5 .mu.m/mK. The silver content in such fire-on alloys compatible with low-melting ceramic must be increased; the higher silver content generally leads, however, to a higher tendency to discoloration of the dental ceramic by silver, which cannot be suppressed even by the solutions quoted in DE-39 05 987.
DE-PS 25 23 971 describes palladium-silver alloys which contain 0.1 to 0.5% of titanium to suppress the ceramic discoloration. Because of the reactivity of titanium with the atmospheric oxygen and/or the crucible material, this element becomes depleted in the melt relatively quickly, so that the reducing effect on the tendency to discolour is lost if old material (feed channels, casting funnels) is used and if melting conditions are unfavourably selected. Titanium also causes a strong adhesion of the embedding compound on the surface of the cast object, which makes dis-embedding and finishing more difficult and more time-consuming.
U.S. Pat. No. 4,350,526 describes palladium-silver alloys which do not have a discolouring effect on dental ceramic because of an addition of 0.1 to 1.0% of silicon. Silicon is insoluble in both palladium and silver. Palladium and silicon also form intermetallic phases so that great brittleness of the alloy and casting fragility can occur.
Like titanium, silicon promotes the reaction with ceramic materials, so that strong adhesion of the embedding compound on the cast object occurs with these alloys also.
According to DE-PS 29 42 373, additions of 0.01 to 5% of silicon, boron and/or germanium should suppress the silver discoloration in dental alloys with 30 to 50% gold contents. Germanium alone cannot, however, bring about such effects in gold-free or low-gold palladium alloys.
NL 9200566 describes fire-on palladium-silver alloys which may also contain indium, tin and zinc in addition to further alloy constituents-. These alloys are provided for facing with conventional high-melting ceramic with a coefficient of thermal expansion around 14.5 .mu.m/mK. In practice, however, it has emerged that these alloys cause strong discolorations on dental ceramic, particularly of the more recent low-melting type with a coefficient of thermal expansion around 16.5 .mu.m/mK.